Insiders call President Barack Obama’s trio of closest foreign policy and security advisers the Holy Trinity, and no one has ever doubted who plays the role of Holy Ghost: counterterrorism czar John Brennan. Whereas National Security Adviser Tom Donilon and his deputy, Denis McDonough, occupy traditional White House roles, Brennan has been Obama’s man in the shadows, as my colleague Michael Crowley detailed in his excellent article on Brennan.

That’s one reason Brennan makes a sensible pick for CIA chief, and Obama will formally nominate him for that job at a 1 p.m. event at the White House today. But Brennan’s backstory suggests that his arrival at Langley, and the reception he receives there, will be something to watch.

Brennan was blocked for the CIA job in Obama’s first term by those on the left who thought he was too close to the prior Administration’s controversial counterterrorism tactics. But in his fallback position at the White House, Brennan has shown himself to be a surprisingly ardent defender of the moral high ground in debates over drone use, the rights of Americans in the war on terrorism and other thorny legal and ethical issues.

That can break both ways at the skittish CIA, where scandal-weary case officers distrust outside judgment of their dark arts but like the cover someone close to the President can provide on morally ambiguous questions of national security.